This invention relates to a multicarrier communication system and a reception device for such a system, and in particular relates to a multicarrier communication system and reception device for same which utilizes interchannel interference (ICI) between the upper and lower two subchannels and the channel of interest.
The bit error rate (BER) in filter bank modulation, DMT modulation, FMT modulation, and other multicarrier communication systems can be improved through the use of received signals comprised by interchannel interference (ICI). Interchannel interference arises from erroneous operation system operation in communication systems, or due to unavoidable environment conditions such as loss of orthogonality between subcarriers. This interchannel interference is caused by spectral energy leakage, and in some cases by a kind of leakage between subchannels called crosstalk.
A turbo-receiver of this invention is based on a maximum posterior probability algorithm utilizing ICI. In this turbo-receiver, information derived from one subchannel after nonlinear processing is refined by the maximum posterior probability of the other subchannel, and similarly, information derived from the other subchannel is refined by the maximum posterior probability of the first subchannel.
[See for example] K. Sathananthan and C. Tellambura, “Probability of error calculation of OFDM system with frequency offset”, IEEE Trans. Commun., Vol. 49, No. 11, November 2001, pp. 1884-1888.
(a) Relation of Frequency Offset to ICI
In a multicarrier communication system in which a frequency band is divided into a plurality of independent narrow subbands, and moreover the transmission data for each subband is frequency-multiplexed, transmitted and received, and in particular in a multicarrier communication system employing filter bank modulation, DMT (discrete multitone) modulation, FMT (filtered multitone) modulation and similar, selection of the filter set has been executed under the constraint of completely eliminating intersymbol interference (ISI) and interchannel interference (ICI).
In an ideal transmission channel in which there is no Doppler shift, there is no offset frequency between transmitter and receiver, and moreover signal distortion does not occur, this constraint guarantees the error-free restoration of transmitted symbols in the receiver. However, a frequency offset in a channel, arising from inaccurate oscillator tuning and Doppler shifts, will in turn cause BER degradation due to spectral leakage or ICI.
The only method for alleviating such BER degradation is to reduce the frequency offset to the extent possible, and specifically, to hold the frequency offset to within 1% of the subcarrier frequency interval. However, this method requires accurate estimation of the frequency offset, and in addition when multicarrier signals are received with noise intermixed, if the noise level is high there is the problem that the accuracy of frequency offset estimation is impaired. Further, in a high-speed fading channel, that is, in a channel in which the Doppler shift is not constant for transmission symbols, and in a high-speed fading channel which changes with time, this method does not operate correctly.
Here, a DMT-base system and an ideal white Gaussian noise channel are supposed. The level of intersymbol interference (ISI) is assumed to be negligible in comparison with interchannel interference (ICI) and other noise signals. To simplify, only the subchannel of interest, a first adjacent subchannel positioned below the subchannel of interest, and a second adjacent subchannel positioned above the subchannel of interest are considered. FIG. 1 and FIG. 2 show the frequency response of the three subchannels cases in which the frequency offset is zero (FIG. 1) and in which the frequency offset is not zero (FIG. 2). The signals of the central frequencies f1, f2, f3 corresponding to the first, second and third subchannels are indicated by vertical arrows in FIG. 1 and FIG. 2. In FIG. 1 and FIG. 2, the subchannel number 0 (ch0) indicates the subchannel of interest, the subchannel number −1 (ch−1) indicates the subchannel positioned below the subchannel of interest on the frequency scale, and the subchannel number +1 (ch+1) indicates the subchannel positioned above the subchannel of interest on the frequency scale. If the DMT symbol period is T, then the frequency scale is normalized by the channel interval, equal to 1/T. That is, one unit of the frequency scale is the channel interval. As shown in FIG. 1, when the frequency offset (normalized by the channel interval) α is 0, the transfer functions of the lower subchannel and upper subchannel, represented by the solid line A and the broken line B in the figure, result in infinite attenuation at the central frequency f2 of the subchannel of interest (dotted line C). Similarly, the transfer function of the subchannel of interest results in infinite attenuation at the central frequencies f1 and f3 of the lower and upper subchannels. That is, if the frequency offset α is zero, then ICI does not occur between adjacent channels. In other words, if the frequency offset is zero, subchannels are orthogonal, and ICI is completely nonexistent.
However, if the frequency offset α is not zero, the subchannel orthogonality collapses and ICI occurs. FIG. 2 shows the spectral characteristics of each subchannel when the frequency offset α is not zero in a DMT system. The spectra of adjacent subchannels clearly have nonzero mutual gains, indicated in FIG. 2 by α0−1, α10, α−10, α01. In this notation, the first index of α indicates the subchannel which is the source of interference, and the second index indicates the subchannel in which interference occurs. That is, α0−1 indicates the leakage transfer coefficient (amplitude) from the subchannel of interest with number 0 to the lower subchannel with subchannel number −1, α10 indicates the leakage transfer coefficient from the upper subchannel with number +1 to the subchannel of interest with subchannel 0, α−10 indicates the leakage transfer coefficient from the lower subchannel with subchannel number −1 to the subchannel of interest with number 0, and α01 indicates the leakage transfer coefficient from the subchannel of interest with number 0 to the upper subchannel with subchannel number +1. In this way, if the frequency offset α is not zero, a nonzero mutual gain, that is, ICI between subchannels (crosstalk) occurs.
(b) Generalized Model of Communication Systems
FIG. 3 is a general model intended to illustrate the mutual ICI between three subchannels in a DMT system having frequency offsets. 11, 12, 13 are transmission devices for the subchannels ch−1, ch0, ch+1; 21, 22, 23 are reception devices for the respective subchannels; 31, 32, 33 are transmission paths for the respective subchannels; 4ij are multipliers to multiply the leakage transfer coefficient (interference coefficient) αij from subchannel number i to subchannel number j by the subchannel signal Di; 51, 52, 53 are synthesis portions which synthesize crosstalk (ICI) from other subchannels with their own subchannel signals; and 61, 62, 63 are noise synthesis portions.
As is clear from FIG. 3, signals from the lower subchannel ch−1 leak into the subchannel of interest ch0 via the crosstalk coefficient α−10, and signals from the upper subchannel ch+1 leak into the subchannel of interest via the crosstalk coefficient α10. The model of FIG. 3 can also be applied to a multicarrier communication system having N subchannels greater than 3, without limiting the number of subchannels in the entire communication system, if the subchannels undergoing mutual interference are limited to the upper and lower subchannels. However, in this case also interference in each of the subchannels is only from the upper and lower adjacent subchannels. In this case, the interference coefficients describe a coefficient chain. Because of frequency orthogonality between subchanhels, the noise components denoted by n1(t), n2(t), n3(t) in FIG. 3 are statistically independent (uncorrelated).
It has been assumed that subchannels are positioned in the frequency domain; however, a similar model can also be applied to other systems, in addition to systems using DMT modulation, filter bank modulation methods and similar.
(c) Technical Problems
The model of FIG. 3 is useful for understanding the physical process which is the cause of ICI. In terms of this model, the problem is to make possible correct determination of the signals received in each subchannel and the values of transmission information symbols (if binary, then codes), even when ICI occurs.
One method which holds the possibility of alleviating ICI in reception devices is adoption of the decision feedback equalizer (DFE) for ICI cancellation proposed in Viterbo and K. Fazel, “How to combat long echoes in QFDM transmission schemes: Subchannel equalization or more powerful channel coding”, Proc. IEEE Globecom '95, Singapore, November 1995, pp. 2069-2074.
However, if the outputs of each reception device are in hard bit decision (hard decision) format, then even if information is shared among subchannels, there is only a very slight advantage. This limits the range of operation of DFE, which uses hard decisions.
Even if the above-described approach is useful in numerous actual cases, benefits depend on the extent to which the ICI effect is minimized. This is because ICI comprises information relating to transmission symbols, and there is the possibility that the transmission symbol information comprised by the ICI can be used in satisfactory demodulation of received signals.